Electrical borehole logging is well known in the field of hydrocarbon (oil and gas) exploration and production, and various devices and various techniques have been described for such purposes. Broadly speaking, there are two categories of devices used in electrical logging. In the first category, transmitter or source electrodes (current sources) are employed in conjunction with receiver or sink electrodes (which may be separate isolated contacts or may comprise part of the mandrel or tool body or an extension thereof). An electrical current flows in a circuit that connects a current source to the transmitter electrodes, through the earth formation to the return electrodes. In a second category, that of inductive measuring tools, one or more antennas of the measuring instrument induces a current flow within the earth formation. The magnitude of the induced current is detected using one or more receiver antennas.
There are different known modes of operation among typical borehole resistivity measurement devices. In one known mode, the current at the measuring electrode is maintained at a constant level and a voltage is measured, while in a second mode, the voltage of the electrode is fixed and the current flowing from the electrode is measured. Ideally, it is desirable that if the current is varied to maintain constant voltage at a monitor electrode, the current is inversely proportional to the resistivity of the earth formation being investigated. Conversely, it is desirable that if the current is maintained constant, the voltage measured at a monitor electrode is proportional to the resistivity of the earth formation being investigated. Those of ordinary skill in the art will appreciate the Ohm's law relation that if both current and voltage vary, the resistivity of the earth formation is proportional to the ratio of the voltage to the current.
Of course, the foregoing is a highly simplified and idealized description of physical relationships that are more complex in practical application. Those of ordinary skill in the art will appreciate that there are usually other variables to consider, for example the electrical characteristics of the instrumentation itself and the electrical characteristics of the environment under investigation. The prior art suggests innumerable approaches for addressing such non-idealized factors.
U.S. Pat. No. 7,365,545 to Itskovich et al., for example, entitled “Two Axial Pad Formation Resistivity Imager,” proposes and arrangement whereby current is injected in two orthogonal directions to assess both horizontal and vertical resistivity in the formation under investigation.
U.S. Pat. No. 6,060,885 to Tabarovsky et al., entitled “Method and Apparatus for Determining the Resistivity and Conductivity of Geological Formations Surrounding a Borehole,” proposes using a plurality of vertically spaced-apart return electrodes in conjunction with a source electrode, in order to provide a resistivity/conductivity profile over an increasing radial depth of investigation.
U.S. Pat. No. 7,385,401 to Itskovich et al., entitled “High Resolution Resistivity Earth Imager,” proposes taking measurements of the standoff distance between an electrode and the borehole wall, in order to account for the electrical characteristics of the drilling fluid (oil based or water based) present between the electrode and the borehole wall. Similarly, U.S. Pat. No. 7,394,258 to Itskovich et al., entitled “High Resolution Resistivity Earth Imager,” proposes taking into account the electrical conductivity and dielectric constant of drilling fluid in the borehole to increase the accuracy of formation resistivity measurements.
U.S. Pat. No. 7,397,250 to Bespalov et al., entitled “High Resolution Resistivity Earth Imager,” contemplates resistivity measurements taken at multiple frequencies to account for tool standoff and the resistivity and dielectric constant of drilling fluid.
U.S. Pat. No. 6,348,796 to Evans et al., entitled “Image Focusing Method and Apparatus for Wellbore Resistivity Imaging” proposes the use of focused electrodes for resistivity measurements.
In the '796 patent, there is disclosed an apparatus that includes an array of measure electrodes separated from a pad or the body of the instrument by focus electrodes, the pad or body acting as the guard electrode. The focus electrode is maintained at a slightly lower potential than the pad, and the measure electrode is at an intermediate potential. With this arrangement, the current from the measure electrode initially diverges as it enters the formation, then converges (focuses), and then finally diverges again to define a depth of investigation. This arrangement is understood to reduce the instruments' sensitivities to borehole rugosity (i.e., variability and uncertainty in the distance between an electrode and the borehole wall).
The use of focusing electrodes is also discussed in U.S. Pat. No. 6,060,885 to Tabarovsky et al., entitled “Apparatus and Method for Wellbore Resistivity Determination and Imaging Using Capacitive Coupling.”
The aforementioned U.S. Pat. No. 7,397,250 to Bespalov et al., U.S. Pat. No. 7,385,401 to Itskovich et al., U.S. Pat. No. 7,365,545 to Itskovich et al., U.S. Pat. No. 6,600,321 to Evans, U.S. Pat. No. 6,348,796 to Evans et al., and U.S. Pat. No. 6,060,885 to Tabarovsky et al., are each incorporated by reference herein in their respective entireties.
In typical borehole resisitivity measuring instruments, such as disclosed in the aforementioned Evans '321 patent or the Itskovich '545 patent, there are a plurality of resistivity arrays, or pads, azimuthally spaced apart around the circumference of the tool body (mandrel). Each array comprises a pad surrounding one or more measurement electrodes (transmit and return) and, possibly, one or more focusing electrodes. There may be, for example, four or six separate resistivity arrays disposed around the circumference of the instrument. As a practical matter, the more arrays provided, the smaller each array must be in order for all arrays to physically fit around the circumference of the array. The fewer arrays provided, the less sensitive the instrument is likely to be to azimuthal variation in the electrical characteristic(s) under investigation.